Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer (for example, a laser beam printer, an LED printer, etc.), and a facsimile apparatus, which forms an image to a recording medium using, for example, an electrophotographic image forming system. In addition, the invention relates to an image forming unit mounted in the image forming apparatus.
Description of the Related Art
In an image forming apparatus, there is left a minute but uncontrollable toner in the vicinity of a developing nip portion where the charged toner is delivered from a developing sleeve to a photosensitive drum. The majority of the toner is conveyed by an air flow and collected and trapped by a filter portion. However, part of the toner is scattered by a viscous flow generated in accordance with the rotation of a photosensitive drum, and attached and accumulated onto members around the developing nip portion without being conveyed to the filter portion.
In this way, when the toner is accumulated on the members around the developing nip portion and the amount of the accumulated toner is increased, the toner accumulates exceeding the angle of repose of the toner and falls down and may cause contamination of an intermediate transfer belt. Then, there is a concern about that the unintended toner is transferred onto a sheet due to the contamination, thereby causing an image defect.
Therefore, there is proposed a method of preventing the intermediate transfer belt from being contaminated when the uncontrollable toner is scattered in the vicinity of the developing nip portion. For example, Japanese Patent Laid-Open No. 2003-156934 discloses a configuration in which a toner receiving tray is provided below the developing nip portion and the toner scattering and falling from the developing nip portion is received into the toner receiving tray.
Herein, a stay for securing a strength of a photosensitive unit may be provided in the configuration in which the photosensitive unit (an image forming unit) which includes the photosensitive drum and a support member to support the photosensitive drum is mounted to a main body of the apparatus so as to form an image forming portion. Then, the stay may be disposed on a downstream side in a rotation direction of the photosensitive drum and above the toner receiving tray from the developing nip portion formed between the developing sleeve and the photosensitive drum when the photosensitive unit is mounted on the main body of the apparatus.
This is because the photosensitive drum and a cleaning unit are generally unitized in the photosensitive unit. Then, the cleaning unit also serves as the stay of the photosensitive unit, and thus the strength of the cleaning unit is easily secured. On the other hand, since a diagonal region from the cleaning unit with the photosensitive drum interposed therebetween is easily decreased in strength, the stay is provided in the region to secure the strength in many cases. Then, when the photosensitive unit provided with the stay at the position is mounted on the main body of the apparatus, the stay is easily disposed on the downstream side in the rotation direction of the photosensitive drum from the developing nip portion.
In this way, in a case where the stay is disposed on the downstream side in the rotation direction of the photosensitive drum from the developing nip portion, the toner scattered from the developing nip portion due to the viscous flow generated according to the rotation of the photosensitive drum may be accumulated on the stay without being received into the toner receiving tray (see FIG. 9A). When the photosensitive drum is rotated in such a state where the scattered toner is accumulated on the stay, the toner accumulated on the stay is likely to go along the viscous flow and slide down so as to fall down onto the intermediate transfer belt without falling down into the toner receiving tray provided below the stay as illustrated in FIG. 9B. Then, in a case where the toner falls down onto the intermediate transfer belt, the unintended toner is transferred onto the sheet. Therefore, the image defect occurs.
Further, in a case where the toner is not accumulated on the stay, as illustrated in FIG. 9C, the toner scattered from the developing nip portion is not blocked by the toner receiving tray. Therefore, the intermediate transfer belt is not contaminated.